This invention relates generally to electrical power conversion systems and, more particularly, to electrical power conversion systems that track the output power of an electrical power source to cause the power source to supply its peak available output power.
Some electrical power sources have the limitation that the maximum output power of the power source occurs at an output current less than its full output current. More specifically, the output power of such power sources increases as the output current increases until an optimum output current is reached whereupon further increases in the output current decreases the output power of the power source. For these power sources, the impedance of the electrical load desirably is controlled so that the power source operates at its optimum current. Often, the peak available output power is related to a variable energy source so that the optimum output current is not constant. Thus, the power load should have a variable impedance that tracks the power source's point of peak output power to allow the power load to consume the maximum available power as the energy source varies.
For example, a solar array manufactured from a plurality of photovoltaic solar cells has an output power defined by the product of its output voltage and output current. The relationship between the output voltage and output current of the solar array, at constant insolation, is well known and is generally defined by a knee-shaped curve. The solar array produces its peak output power near the corner of the knee-shaped curve, where the product of its output voltage and output current is maximized. If the current is increased past the optimum output current, such that the output current approaches the solar array's maximum or full output current, the solar array's output voltage decreases more rapidly than the output current increases. This sharp voltage reduction decreases the product of the solar array's output voltage and output power, causing a net decrease in the solar array's output power.
The solar array's actual operating output voltage and output current is defined by the impedance of the electrical power load. Thus, the power load's impedance usually is set to maximize the solar array's output power.
It is well known that changes in the insolation collected by the solar array and in the solar array's temperature cause the knee-shaped curve of the solar array to shift. Thus, the power load's impedance should be adjustable to accommodate any changes in the solar array's peak available output power. More specifically, on a sunny day at noon, a fixed power load impedance typically is set to cause the power load to draw the maximum available output power from the solar array. However, on a cloudy day, the maximum available output power will be reduced. In addition, the power load will draw less than the maximum available power output of this reduced output power unless its load impedance is appropriately adjusted to track the shift in the peak operating point of the solar array.
The size of the solar array is a significant factor in determining the cost of the electrical energy delivered by a solar electrical power source. Tracking the solar array's peak operating point allows for more efficient use of the solar array which lowers the cost of the electrical energy.
Accordingly, solar battery charging systems or the like have been developed that adjust the impedance of the power load connected to the solar array to track the peak of the available output power as the insolation on the solar array changes. Frequently, the power load is a dc-to-dc converter that varies the current drawn from the solar array until the impedance of the dc-to-dc converter causes it to draw the maximum available power from the solar array.
Often, the electrical power load is a well-defined load of known characteristics and the power converter is designed to interface a specific power source with a specific electrical power load. For example, the power source may be a solar array and power load may be a battery or an electrical motor.
However, it may be desired to have the solar array supply power to many separate and removable power loads or to several different load types. For example, it may be desired to have the solar array configured to simultaneously charge several different battery types or to power a mix of batteries and motors. Also, it may be desired to disconnect one or more of the power loads from the solar array. Controlling the system so as to draw the peak available power therefore becomes substantially more complicated.
From the foregoing, it should be apparent that there is a need for an electrical power conversion system that can supply electrical power to a plurality of varied and removable electrical power loads while controlling the current draw of the power loads such that, in combination, the power loads consume the maximum available power from a variable power source such as a solar array. The present invention satisfies this need.